1. Field of the Invention
The present invention concerns a device for floor heating comprising a bendable, electrically conductive, thermoplastic mat, adapted for division into lengths and mounting of these lengths side by side underneath a floor and provided with at least two electrodes arranged along each side edge of the mat, to which electrodes a current is connectable, whereby the current is conducted through the device, which heats up and emits heat and whereby the mat comprises a material composition whose volume resistivity increases when the temperature of the material composition increases.
2. Description of the Related Art
Previously known devices for floor heating, xe2x80x9cStep warmfloorxe2x80x9d and xe2x80x9cEP-vxc3xa4rmegolvxe2x80x9d, comprises a heat mat, stored in a coiled up position, of an electrically conductive, thermoplastic material wherein the volume resistivity progressively increases with the temperature. This phenomena is called, in the art, Positive Temperature Coefficient (PTC). The heat mat comprises two electrodes extending along the edges to which a current may be connected, which is conducted through the heat mat and causes a heating of the mat. When the volume resistivity for the material increases with increased temperature, the resistance in the heat mat increases which in turn causes the power to decrease and thereby also the temperature even though the voltage is constant. The heat mat is cut into preferred lengths, connected electrically to each other and mounted side by side underneath a floor.
A problem with the heat mat of xe2x80x9cStep Warmfloorxe2x80x9d and xe2x80x9cEP-vxc3xa4rmegolvxe2x80x9d is that it may only be driven by low voltage. In their cases the working voltage is 24 V. That means that at least one transformer is needed and the electrodes must comprise braided copper threads to be able to supply the mat with sufficient amount of current.
Another problem with the heat mats of xe2x80x9cStep Warmfloorxe2x80x9d and xe2x80x9cEP-vxc3xa4rmegolvxe2x80x9d is that the material making up the mat does not increase its volume resistivity pronouncely until the material reaches about 55xc2x0 C. This brings about a risk of too hot floors, especially if the floor is covered by a matress or a thick carpet, whereby for example a child may get burnt when the matress or carpet is removed. Additionally, wooden floors dry out and will be damaged by too high temperatures. In other words the heat mats of xe2x80x9cStep Warmfloorxe2x80x9d and xe2x80x9cEP-vxc3xa4rmegolvxe2x80x9d are not self regulating regarding temperature and power in the temperature range which is commonly applicable for floorheating. The heat mats of xe2x80x9cStep Warmfloorxe2x80x9d and xe2x80x9cEP-vxc3xa4rmegolvxe2x80x9d are used without insulation.
An object of the present invention is to provide a device for floorheating, which may be driven by mains voltage.
Another object of the present invention is to provide a device for floor heating which is self regulating for temperatures suitable for floor heating i.e. that the volume resistivity increases pronouncely at these temperatures, which also results in an overheating protection.
A further object of the present invention is to provide a device for floor heating, which quickly and easily may be installed.
These objects are met with a device according to the preamble, which is characterised in that the electrodes are threadlike and that the material composition comprises a semicrystalline polymer, whose crystalline melting point exceeds the normal working temperature of the device but is below 99xc2x0 C. and whose softening temperature (Vicat) is in the range of 45xc2x0-70xc2x0 C., and an electrically conductive filler material, which results in that the device may be driven by mains voltage and that the volume resistivitiy of the material composition substantially increases already at temperatures below 30xc2x0 C., whereby the device is self regulating regarding temperature and power at normal working temperatures for floor heating.
Similar devices have been on the market for some time but never for another use except for low voltage. The understanding of making an electrically conducting material for mains voltage, which at the same time is self regulating for temperatures suitable for floor heating serves as a basis for the invention.
The semicrystalline material may be a copolymer, for example an ethylene-ethyl acrylate polymer (EEA), ethylene-butyl acrylate polymer (EBA), ethylene-methyl acrylate polymer (EMA), ethylene-vinyl acetate polymer (EVA) or a so called plastomer. Preferably, the crystalline melting point should be in the range of 75-99xc2x0 C. or rather in the range of 80-96xc2x0 C. and the electrically conductive filler material should be carbon black.
The mat may be provided with a fully covering electrically insulating layer so that the device is ready to be used without further insulation or any risks of electric flash-over. Preferably the electrically insulating layer is coextruded with the electrically conductive mat, but may also be provided in other ways, such as foliating.
The electrically insulating layer may, for example, be a low density polyethylene (LDPE), linear low density polyethylene (LLDPE), polypropylene (PP), polyester or any of the previously mentioned copolymers, which preferably should have a higher melting point than the semicrystalline material of the mat. Also some plastomers are conceivable. The electrically insulating layer may comprise a plurality of layers of different materials, which may have different purposes, for example an inner layer for good adhesion to the mat, a middle layer for building up a thickness and an outer layer for better scrath resistance, for example, firmly attached to each other. Preferably, electrically insulating filler material, for example titanium dioxide, chalk or silica, is added in the insulating layer. Also flame retarding materials, such as aluminium tri hydrate and/or magnesium hydroxide, may be added. Both electrically insulating and flame retarding materials yield improved mechanical properties.
In order to easily mount the mat in, for example, concrete or clinkers concrete, the mat may be provided with through holes. These holes may have any shape, for example slot shape or rectangular or circular holes, but ought to be arranged in rows perpendicularly to the extension of the mat so that the current passage between the electrodes will not be disturbed. Preferably the through holes are electrically insulated along their edges.
An advantage of the present invention is that there is not any need for transformers. Another advantage is that less copper is used since a single copper thread or a copper wire is sufficient as an electrode to supply enough of the current at mains voltage. A further advantage is that smaller electrical fuses are required. Overall, the inventive device will therefore be less expensive.